Functionnal analysis (norm in Sobolev space)

[quasar987]

[SOLVED] Functionnal analysis (norm in Sobolev space)

Homework Statement

In relations to the problem of finding the eigenvalues of the operator -d²/dt², it can be shown (Lax-Milgram) that given f in L²[0,1], there exists a unique u in H^1_0[0,1] such that

\int_0^1u'v' = \int_0^1fv \ \ \forall v \in H^1_0[0,1]

Consider the map S:L²[0,1]-->H^1_0[0,1] defined by S(f)=u.

In arguing that S in continuous, my book says to take v=u. The equation characterizing u is then

\int_0^1(u')^2 = \int_0^1fu

We deduce from this that ||u'||^2_{L^2}\leq ||f||_{L^2}||u||_{L^2} (Hölder). And here my book simply says "It results that ||u||_{H^1}\leq C||f||_{L^2}, for a constant C."

How does that follow?

Homework Equations

||u||_{H^1}=\sqrt{||u'||_{L^2}^2+||u||_{L^2}^2}

The Attempt at a Solution

Well, we can use the inequality to write

||u||_{H^1}^2=||u'||_{L^2}^2+||u||_{L^2}^2\leq |f||_{L^2}||u||_{L^2}+||u||_{L^2}, but then what?

 

[quasar987]

That last equation should read

||u||_{H^1}^2=||u'||_{L^2}^2+||u||_{L^2}^2\leq ||f||_{L^2}||u||_{L^2}+||u||_{L^2}^2

 

[quasar987]

Perhaps slightly closer would be to use

ab\leq \frac{a^2+b^2}{2}

on Hölder's inequality to obtain instead

||u'||^2_{L^2}\leq \frac{||f||_{L^2}^2+||u||_{L^2}^2}{2}

and thus

||u||_{H^1}\leq \frac{3}{2}||u||_{L^2}^2+\frac{1}{2}||f||_{L^2}^2

But can we relate ||u||_2 to ||f||_2 ?